This invention relates to a new process and apparatus for producing generally elongated composite articles of fiber-reinforced thermosetting resins.
More specifically, this invention provides a novel process for continuously producing composite articles of cylindrical shape or other elongated shapes of fiber-reinforced thermosetting resins, which process, unlike the known processes, makes high-speed production of such articles possible.
As a continuous process for producing composite articles of fiber-reinforced thermosetting resins, a so-called pultrusion process, in which a fiber reinforcing material in roving or mat state or a composite structure thereof is impregnated with a thermosetting resin, and the resulting mixture is introduced into a heated die for shaping and setting or curing having a passage of the ultimate product shape thereby to shape and set or cure the same, has heretofore been proposed.
For increasing the production speed in the pultrusion process, the method of elevating the temperature of the setting die and the method of elongating the length of the setting die are conceivable. However, if the temperature is raised too abruptly in the former method, the balance between the raise in viscosity of the resin accompanying its setting and the vaporization of the diluent will collapse, which will give rise to problems such as foaming or early setting of only the outermost layer contacting the die, whereby a great viscosity gradient develops between the outer layer and the inner layer, as a result of which a high internal stress is created in the product and gives rise to a deterioration of the product performance. Furthermore, by the latter method, the length of intimate contact of the resin against the inner surface of the die due to thermal expansion of the resin at the time of its heating becomes long, and, as a result, the resistance to the pultrusion is increased, whereby the length of the die is practically limited.
As a consequence, the present state of the art in pultrusion is such that it is being practiced at a production rate which is much slower than that practiced in general extrusion of thermoplastic resins.
In view of this situation, proposals have been made for solving the above described problems, one of which is disclosed in Japanese Patent Laid Open Publn. No. 8865/1975. The process of this proposal comprises preparing a mixture of a reinforcing fiber material impregnated with a thermosetting resin, coating this mixture with thermoplastic resin in molten state, immediately introducing the coated mixture into a heating air vessel or a hot-water tank, and, as the mixture is shaped by means such as guide rolls, causing it to set. While this proposed process solves the problems of the known pultrusion process, it is accompanied by new problems relating to high-speed forming.
More specifically, in the case where an air vessel is used, if the temperature is raised too much, the coating resin forming the outer surface will deteriorate. In the case of a hot-water tank, there is an upper limit to the temperature of substantially 97.degree. to 98.degree. C.
Even if, at the risk of deterioration of the coating resin, the temperature of the air vessel is raised, or high-temperature heating is sought by using a sealed steam vessel, when the temperature of the coating resin is increased above its melting point, molten resin will adhere to parts such as guide rolls for shaping, or deformation will occur. For this reason, there will still be an upper limit to the temperature, whereby there is a limit to high-speed forming.
Another process is disclosed in Japanese Patent Publn. No. 43501/1976. This proposed process comprises preparing a mixture by impregnating a reinforcing fiber material with a thermosetting resin, coating this mixture with a molten thermoplastic resin, cooling and solidifying the coating surface resin layer, and producing at high speed the intermediate composite material with its interior being unset. The final product is produced by setting this intermediate composite material by a separate suitable heating process.
The most important characteristics of this proposed process are the possibility of setting the unset intermediate material in a state wherein it is shaped in the final product shape such as an arcuate shape or a coil shape and the fact that, by separating the process into the two steps of producing the unset intermediate material and setting, increasing the production rate of each process step has become possible.
However, this process involves cooling a material which has been once heated and then reheating the same, whereby this forming process has the problem of much loss of energy, and it cannot be said to be a desirable process from the standpoint of energy saving.